1. Field of the Invention
The invention relates generally to atomized non-ferrous metal powders and particularly to an air atomized prealloyed powder comprised of copper, tin and zinc and having high green strength in the "as atomized" form.
2. Description of the Prior Art
Certain non-ferrous powders available for powder metallurgical usage are formed according to a variety of techniques, the prior techniques requiring multi-step processing in order that a necessary degree of "green strength" be imparted to the powder. Such powders are subjected to compacting pressures in forming dies, the "cold compact" thus formed being required to exhibit a given degree of mechanical integrity after compacting in order to allow physical movement from the compacting press equipment to the trays or belts employed in sintering operations performed in a furnace. In order that a metal powder be capable of such mechanical integrity, the green strengh of the powder must be of a given degree. In U.S. Pat. No. 3,888,657, for example, acceptable green strength is imparted to semi-irregular powders by thermal sintering and agglomeration of powder fines to form a sinter cake. The cake or agglomerated mass is then disintegrated or broken down to fine particles, such as by milling, the fine particles consisting of agglomerated fines. The semi-irregular powders thus formed have a green strength adequate to produce a suitable "cold compact" necessary to allow further processing.
In the aforesaid patent, it is indicated that metal powders formed by atomization have poor green strength. The green strength of "as atomized" metal powders treated according to the above-described process of said patent was increased by a factor of ten over the relatively poor green strength of the atomized powder. The present invention provides a prealloy powder which in the "as atomized" condition does not require further processing or modification by secondary operations to impart a desired high degree of green strength to the powder.
In U.S. Pat. No. 3,832,156, metal powders formed of either spherical copper or prealloy bronze and having extremely low green strength in the "as atomized" forms were converted to powders of an irregular form to increase the green strength thereof. This conversion was effected by secondary operations, such as ball milling, annealing and disintegration, the secondary operations being necessary to impart sufficient green strength to the "as atomized" metal powders to allow for the compacting and sintering of useful articles. The necessary green strength provided to the "as atomized" metal powders was accomplished by modification of particle shape only by the use of secondary operations performed after atomization of the powders into a relatively low green strength form. The present invention provides a prealloy powder of copper, tin and zinc which has sufficient green strength in the "as atomized" form to allow for the immediate compacting and sintering of useful articles without the need for modification of particle shape by secondary mechanical conversion.